Facile synthesis of gold nanocages with silver nanocubes templates dual metal effects enabled SERS imaging and catalytic reduction

Silver (Ag) nanomaterials featuring a cubic shape particularly represent supreme class of advance nanomaterials. This work explored a new precursor and its effect on morphological features of silver (Ag) nanocubes (NCs) serving as sacrificial templates for facile synthesis of gold NCs. The AgNCs were initially prepared utilizing sodium thiosulphate (Na2S2O3) as relatively stable S2− producing species along with a soft etchant source KCl. The effects of different potassium halides were evaluated to grasp control over seed mediated growth of Ag nanocubes. Taking the advantages of dual metallic properties, Ag@4MBA@AuNCs nanostructure was synthesized using 4-mercaptobenzoic acid (4MBA) as a Raman reporter molecule. This nanostructure showed 1010-times enhancement in surface enhanced Raman scattering (SERS) signal, leading to a highly sensitive imaging probe for the detection of even three breast cancer cells (MCF-7 cells) in vitro. Subsequently, the oxidative nanopeeling well accompanied by incorporation of Au/Ag alloy nanoparticles on AuNCs corona assembly was achieved, which facilitated the catalytic reduction of toxic nitrophenol to eco-friendly aminophenol. Such sophisticated and engineered nanoassemblies possess broad applications in bioanalysis.

Instrumentation.Transmission electron microscopic (TEM) images and energy dispersive Xray spectra (EDS) were recorded on Tecnai F20 transmission electron microscope operated at 200 kV.Surface morphology and element distribution of Ag nanocubes and AuNCs were recorded on field emission scanning electron microscope (FESEM, S-4800).FTIR spectra of samples were recorded on Nicolet Is10 (USA) spectrometer.For powder X-ray diffraction (XRD) of the samples Rigaku Rotaflex Dmax2200 diffractometer (Japan) with Cu Kα radiation (λ = 1.54056Å) was used.The elemental composition and chemical valence states of Ag nanocubes and AuNCs were analyzed by X-ray photoelectron spectroscopy (XPS) recorded on Kratos Axis Ultra DLD with a nominal energy resolution of 0.48 eV at room temperature.Absorption spectra were recorded on UV-3600 UV-Vis-NIR spectrophotometer made by Shimadzu, Japan.Raman and SERS spectra were obtained with Renishaw in Via Reflex instrument, England.Lasers 633 and 532 nm lines from an Ar-Kr ion laser, and laser 785 nm line from semiconductor laser were used as excitation sources.The cell viabilities in the MTT assay were measured through a BioTek ELX 800 microplate reader.
Calculation of Enhancement Factor.We used the SERS peak at 1077 cm -1 of 4MBA reporter molecule to calculate the SERS enhancement factor (EF) using the following equation: I SERS and I bulk are the intensities of the same band for the SERS and ordinary spectra from a bulk sample, N bulk and N SERS are the numbers of bulk molecules probed for a bulk sample and those molecules probed in SERS, respectively.
Note that I SERS and I bulk were determined by the area of 1077 cm -1 bands while N bulk was determined from the ordinary Raman spectrum of a 0.01mM 4MBA solution and the focal volume of our Raman system.Specifically, we evaluated the EF by using 4MBA@AuNCs and Ag@4MBA@AuNCs.Based on the SERS spectra shown in Figure 3, we measured the I SERS .For determining the number of molecules probed in SERS, we assumed that the surface was covered by a complete monolayer of 4MBA molecules.Hence, theoretically the calculated value provides a maximum number of molecules and thus calculated EF values necessarily represent an underestimate of the actual value.Ag@4MBA@AuNCs at various concentrations for 24 h recorded at 550 nm.

Figure S3 .
Figure S3.SEM image and XRD spectrum of as synthesized Ag nanocubes.

Table S1 :
Effect of KCl concentration on LSPR of Ag nanocubes.

Table S2 :
Optimization of Na 2 S 2 O 3 and salt concentration for synthesis of Ag nanocubes.